Modern well drilling techniques, particularly those concerned with the drilling of oil and gas wells, involve the use of several different measurement and telemetry systems to provide petrophysical data and data regarding drilling mechanics during the drilling process. Data is acquired by sensors located in the drillstring near the bit and either stored in downhole memory or transmitted to the surface using MWD telemetry devices. Prior art discloses the use of downhole devices incorporating resistivity, gravity, magnetic and nuclear magnetic resonance measurements on a rotating drillstring.
Prior art devices are limited to measurement devices that rotate with the drillstring. This is particularly problematic in nuclear magnetic resonance (NMR) measurements where lateral vibrations of a drill collar containing the NMR device would adversely affect an NMR measurement. For example, a lateral, 50 Hz vibration of 1-mm amplitude (100-g acceleration) would disable a typical device with a resonance region of the order of 1 mm. Furthermore, since the drillstring can make anywhere between 0.1 to several rotations in the duration of a pulsed NMR measurement (on the order of 0.01 to 1 second), an NMR device on a drillstring must be rotationally symmetric. Prior art NMR devices in which the static magnetic fields are produced by magnets located in the drilling collar suffer from the additional disadvantage that the resonance region extends into the borehole, as a result of which an electromagnetic signal is produced in the borehole fluid. The resulting electromagnetic signal of the borehole fluid must be canceled because the pulsed NMR device functions by detecting protons in fluids. Typically, a porous rock formation may contain 10% fluid by volume whereas the borehole fluid contains more than 50% fluid and has a high density of protons. As a result of this, the electromagnetic signal of the borehole fluid would dominate any formation signal detected by the pulsed NMR device and a special arrangement is necessary to cancel the borehole fluid signal. The present invention overcomes these inadequacies.